In agile development there is much discussion about the optimum length of an iteration (or Sprint in the Scrum agile management methodology).

Although Scrum prescribes 30 days, in a recent post about agile development principles, "Fast but not so furious!", I asserted that there is no perfect length for any situation; that it depends on individual circumstances and the nature of the application.

Although it's not necessarily possible to release the software after each iteration, it is widely considered that 1 week is the shortest and 1 month is the maximum Sprint length to retain many of the benefits of an iterative, agile development approach.

Mike Cohn is one of the key exponents of agile development and has a great article about this topic on his web site: "How to select optimum agile development iteration length"

For further reading about agile principles, please see 10 Key Principles of Agile Software Development.